Korean deposits have a huge range depending on the location and type of property. Generally, the higher the deposit, the less monthly rent you have to pay.
When I was looking for housing in Seoul, the lowest housing deposit I could find for a decent one-room was 5,000,000. However, there really weren’t too many options and housing is getting more expensive. I would aim for a 10,000,000 won deposit.
As for paying you back at the end of the contract. I have never heard of anyone not getting their deposit back. If they happened you could contact the Seoul Global Center, or your local foreigner office and they would assist you with finding a lawyer.
Like what Kizzy said, the amount is vary depends on the property type and owner. Usually owner will fully return the deposit to you when your contract ended except if there is any problem with the house when you hand over to the owner that is caused by you, then you need to pay the owner for the repair fees etc. If you don’t pay the owner, then they have the rights deduct from the deposit before returning to you if I am not wrong.
I have the same concern about it, hahaha. I also have a friend that found a place close to Sillim Station that she paid $1.000.000 for deposit! If you guys know somewhere like this, let’s share info hahha